Because of excellent magnetic properties, Nd—Fe—B permanent magnets find an ever increasing range of application. To meet the recent concern about the environmental problem, the range of utilization of magnets has spread to cover household appliances, industrial equipment, electric automobiles and wind power generators. This requires further improvements in performance of Nd—Fe—B magnets.
Typical indices of magnet performance are remanence (residual magnetic flux density) and coercive force. The remanence of Nd—Fe—B sintered magnets can be increased by increasing the volume fraction of Nd2Fe14B compound and improving the orientation of crystal grains. Heretofore, many improved processes have been proposed. With respect to the increase of coercive force, there have been proposed many approaches including refinement of crystal grains, use of alloy compositions with increased Nd contents, and addition of effective elements. The current most common approach is to use alloy compositions in which Nd is partially replaced by Dy or Tb. By substituting Dy or Tb for some Nd in Nd2Fe14B compound, the compound is increased in both anisotropic magnetic field and coercive force. On the other hand, the substitution with Dy or Tb results in the compound having reduced saturation magnetic polarization. Therefore, as long as it is intended to increase the coercive force by this approach, a lowering of remanence is inevitable. Additionally, since Tb and Dy are expensive metals, it is desirable to minimize the amount of Tb and Dy used.
In Nd—Fe—B magnets, the magnitude of an external magnetic field, which creates the nuclei of reverse magnetic domains at grain boundaries, provides a coercive force. The nucleation of reverse magnetic domains is largely affected by the structure of grain boundary, and a disorder of crystalline structure adjacent to the boundary or interface induces a disorder of magnetic structure and facilitates formation of reverse magnetic domains. Although it is generally believed that a magnetic structure extending from the grain boundary to a depth of approximately 5 nm contributes to an enhancement of coercive force, it is difficult to produce an effective form of structure for coercive force enhancement.
Japanese Patent No. 3,471,876 discloses a rare earth magnet having improved corrosion resistance, comprising at least one rare earth element R, which is obtained by effecting fluorinating treatment in a fluoride gas atmosphere or an atmosphere containing a fluoride gas, to form an RF3 compound or an ROxFy compound (wherein x and y have values satisfying 0<x<1.5 and 2x+y=3) or a mixture thereof with R in the constituent phase in a surface layer of the magnet, and further effecting heat treatment at a temperature of 200 to 1,200° C.
JP-A 2003-282312 discloses an R—Fe—(B,C) sintered magnet (wherein R is a rare earth element, at least 50% of R being Nd and/or Pr) having improved magnetizability which is obtained by mixing an alloy powder for R—Fe—(B,C) sintered magnet with a rare earth fluoride powder so that the powder mixture contains 3 to 20% by weight of the rare earth fluoride (the rare earth being preferably Dy and/or Tb), subjecting the powder mixture to orientation in a magnetic field, compaction and sintering, whereby a primary phase is composed mainly of Nd2Fe14B grains, and a particulate grain boundary phase is formed at grain boundaries of the primary phase or grain boundary triple points, said grain boundary phase containing the rare earth fluoride, the rare earth fluoride being contained in an amount of 3 to 20% by weight of the overall sintered magnet. Specifically, an R—Fe—(B,C) sintered magnet (wherein R is a rare earth element, at least 50% of R being Nd and/or Pr) is provided wherein the magnet comprises a primary phase composed mainly of Nd2Fe14B grains and a grain boundary phase containing the rare earth fluoride, the primary phase contains Dy and/or Tb, and the primary phase includes a region where the concentration of Dy and/or Tb is lower than the average concentration of Dy and/or Tb in the overall primary phase.
These proposals, however, are still insufficient in producing a sintered magnet having high performance in terms of remanence and coercive force while reducing the amounts of Tb and Dy used.
JP-A 2005-11973 discloses a rare earth-iron-boron base magnet which is obtained by holding a magnet in a vacuum tank, depositing an element M or an alloy containing an element M (M stands for one or more rare earth elements selected from Pr, Dy, Tb, and Ho) which has been vaporized or atomized by physical means on the entirety or part of the magnet surface in the vacuum tank, and effecting pack cementation so that the element M is diffused and penetrated from the surface into the interior of the magnet to at least a depth corresponding to the radius of crystal grains exposed at the outermost surface of the magnet, to form a grain boundary layer having element M enriched. The concentration of element M in the grain boundary layer is higher at a position nearer to the magnet surface. As a result, the magnet has the grain boundary layer in which element M is enriched by diffusion of element M from the magnet surface. A coercive force Hcj and the content of element M in the overall magnet have the relationship:Hcj≦1+0.2×M wherein Hcj is a coercive force in unit MA/m and M is the content (wt %) of element M in the overall magnet and 0.05≦M≦10. This method, however, is extremely unproductive and impractical.